A conventional steel golf shaft can be described as a tapered tube comprising three main sections: a grip section, a mid-section and a tip section.
The grip section comprises a parallel (in cross-section) tube with typical sizes ranging from 0.560 to 0.625 of an inch in diameter. During the club assembly process the grip section is cut to length prior to being covered in a rubber or leather grip that the player will hold.
The mid-section is the tapered part of the shaft. This section can comprise either a series of discrete steps or reductions in diameter, a constant or “smooth” plain taper, or a combination of both. The rate at which the diameter is reduced in this section is directly related to the overall stiffness of the shaft.
The tip section is generally parallel, and approximately 0.370 of an inch in diameter, and can vary in length depending on the stiffness requirements and the length of the club. In general, the tip section of a shaft is the longest for a driver and the shortest for a wedge. The diameter of the tip section is directly related to the shaft stiffness and playability. The larger the diameter the stiffer the shaft will play; conversely the smaller the diameter the softer or more flexible the shaft will play. When designing golf shafts the relationship between the length and diameters of both the tip and grip sections determines the overall playing characteristics of the shaft.
In addition to the outer geometry, the wall thickness of the shaft is also tightly controlled and has a significant influence on the overall performance of the shaft. Wall thickness is directly related to the overall weight of the shaft and can be manipulated to change both the bending stiffness and balance point of the shaft.
An exemplary conventional “stepped” steel golf shaft is illustrated in FIG. 1. Starting from the upper end of the mid-section adjacent the grip end, the exemplary steel golf shaft may have a first series of four reductions of 0.020 of an inch each, spaced approximately 1.75 inches apart, and a second series of ten reductions of 0.015 of an inch each, spaced approximately 1.5 inches apart.
The diameter reductions are achieved through the use of a plurality of step dies each further reducing the diameter along the shaft from the butt end to the tip end. Due to limitations of both the process and material, step tapers are generally limited to diameter reductions equal to or greater than 0.010 of an inch and cannot be spaced consistently and reliably at less than about 0.50 of an inch apart.
The taper steps are generally located along the mid-section of the shaft in a manner and location that achieves specific flexural characteristics of the shaft.
In an optional secondary swage process, these steps can be blended into the overall taper of the shaft so that the individual taper steps are no longer visible.
As explained in U.S. Pat. No. 5,989,133, the conventional taper press operation consists of a process whereby a steel tube is pushed through a series of support bushings and then into a forming die of a smaller diameter. When removed from the die, the formed part of the tube maintains the smaller diameter. This operation is repeated on a rotary taper press machine that can place a series of steps on one shaft. The taper press operation is a very consistent and reliable method of forming diameter drops of between 0.010″ and 0.025″. However, it has limited use when forming smaller reductions in diameter as the reduced forming loads can allow the tube to be pulled to one side resulting in a non-concentric half step. Due to the limited number of forming barrels there is also a limit to the number of steps that can be formed, which negates the possibility of forming a series of very small steps in an efficient manner.
As noted above, it is also known within the golf industry to use a swaging process to form so-called plain taper or “stepless” steel golf shafts. The swaging process works by forcing a pre-formed or blank tube into a set of half dies that are rotated while opening and closing at very high speed. The dies themselves have the final form cut into their working surfaces, so that when the dies are closed they form a cavity to which the tube conforms. Shafts formed via the swaging process provide a generally constant taper rate that result in a shaft having a smooth or “featureless” appearance.
It is an object of the present invention to provide a steel golf shaft having a loading zone that is selectively positioned along the length of the shaft and formed by a plurality of closely spaced “micro-steps,” or drops in diameter, that modify the bending and playing characteristics of the shaft in a variety of predictable ways. In addition, it is a further object of the present invention to provide a unique process for producing such a golf shaft.